Are physiological effects triggered by CO₂ narcosis mediated through anoxia?

Carbon dioxide (CO₂) is a commonly used anesthetic for insect immobilization but is also used to induce transition to reproduction in bees. In Bombus impatiens queens, CO₂ increased ovarian activation, aggression,and depletion of fat body lipids, however, it is unclear if the impacts are the same in workers, and if these effects are directly caused by CO₂ or whether they are mediated by the lack of oxygen. Here, we examined the physiology and gene expression of workers, mated and unmated queens following exposure to anoxia, hypoxia, full and partial hypercapnia, as compared to untreated controls. All treatments, but hypoxia and control, caused immobilization of the bees. Upon recovery from hypercapnia, but not from anoxia, females showed increase in abdominal pumping, however, queens were immobilized for a longer period than workers. Hypercapnia, either partial or full, resulted in increased ovary activation and decreased lipid amounts in queens but not in workers, with a larger impact caused by full hypercapnia. Also, the anoxia treatment resulted in inactivated ovaries in mated queens, but not in unmated. These results show that CO₂ has a caste-specific effect in bumble bees. Whether CO₂ impact is mediated through the lack of oxygen depends on the mating status of the queens. This study contributes to our understanding of the mechanistic basis of CO₂ narcosis in bees.